(1) Field of the Invention
The present invention relates to an optical transmitting apparatus and an optical transmitting method for a ring transmission system suitable for use in a ring transmission system in which a plurality of optical transmitting apparatuses are connected to one another over a bidirectional ring transmission path having a data link channel.
(2) Description of Related Art
In optical transmission using optical fibers, signal multiplexing is done with SDH (Synchronous Digital Hierarchy) in order to improve the reliability of transmission lines, improve services, and make management easy. In concrete, the optical transmission uses architecture called SONET (Synchronous Optical Network). SONET is a high-speed leased line service adopting SDH, where a plurality of optical transmitting apparatuses are connected to one another through optical fibers to configure a transmission path in a ring form (hereinafter referred to as a ring transmission path, or merely a ring occasionally). Optical signals are transmitted in the ring-formed transmission system. On the ring transmission path, optical signals are transmitted as a plurality of time slots having been time-division-multiplexed, and a system called OC48 (Optical Carrier 48), or OC192 (Optical Carrier 192) is adopted as its transmission mode.
In OC48, 48 time slots are time-division-multiplexed, thus 48 lines are involved, whereas in OC192, 192 time slots are time-division-multiplexed, thus 192 lines are involved. A half of these multiplexed time slots are assigned to lines for working (hereinafter referred to as working lines), and the remaining half thereof are assigned to lines for protection (hereinafter referred to as protection lines). Incidentally, the working line is indicated as Working or Work, whereas the protection line is indicated as Protection, PT or PTCT.
In addition, the multiplexed time slots have a control channel called a data link channel so that the optical signals are properly transmitted to the destination. In the following description, the data link channel is occasionally referred to as a data link.
As well known, in the ring transmission system, an optical transmitting apparatus inserts an optical signal (adds an optical signal) to the ring transmission system, and the added optical signal is transmitted over optical fibers. Another optical transmission apparatus extracts the transmitted optical signal (drops the optical signal), whereby the optical signal is transferred in the ring transmission system. Namely, the data link permits add/drop information to be transmitted, which in turn allows the optical transmitting apparatuses connected through the optical fibers to read and write contents of the data link.
One line of the multiplexed time slots is called an STS line, corresponding to one structural unit of OC48 or the like. In concrete, a line referred to as STS-1 (Synchronous Transport Signal Level 1) is used. The format relating to a speed of the STS-1 line specifies a speed of 51.840 Mbit/s. OC48 therefore has a speed of 2.4 Gbps (51.840 Mbit/s×48) since 48 time slots are multiplexed.
In OC48, there are two types of structures to transmit optical signals on multiplexed 48 lines; one is two-fiber BLSR structure using one optical fiber, the other is four-fiber BLSR structure using two optical fibers. In concrete, the two-fiber BLSR structure assigns a half of the transmission capacity to a bandwidth for the working lines, assigning the other half to a bandwidth for the protection lines. In the four-fiber BLSR structure, one optical fiber is assigned to a bandwidth for the working lines, whereas the other one is assigned to a bandwidth for the protection lines. Recent use of four-fiber BLSR allows a bandwidth for working lines to be largely secured, leading to effective use of the bandwidth of the working lines. Four-fiber BLSR also permits more complicated line settings and line switching than two-fiber BLSR.
For transmission of optical signals in the ring, there are a UPSR (Unidirectional Path-Switched Ring) structure capable of transmission in only one direction, and a BLSR (Bidirectional Line-Switched Ring) structure capable of transmission in both directions. The BLSR structure becomes the mainstream, by which STS (Synchronous Transport Signal) line can be effectively used.
BLSR has two modes; normal BLSR and submarine BLSR, as will be described later.
FIG. 58 is a schematic diagram showing the UPSR structure. A ring transmission system 80 shown in FIG. 58 can transmit optical signals in only one direction. The ring transmission system 80 has five optical transmitting apparatuses (nodes) given respective node IDs (Node Identifications) A, B, C, D, and E, where an optical signal added to the node B is passed through a route along the node B and the node C, and dropped from the node D, while passed through the node A and the node E, and dropped from node D. Here, the node is the same as the optical transmitting apparatus. The optical transmitting apparatus will be occasionally referred to as a node when description is made of a ring connection in the following description. In the ring transmission system 80, transmission of optical signals is interrupted when the optical fiber between the node A and the node B, or the node A and the node C is cut due to occurrence of any failure (line failure).
FIGS. 59(a) through 59(c) are schematic diagrams of the BLSR structure, in which two lines, a working line and a protection line, are provided among nodes. A ring transmission system 81a shown in FIG. 59(a) is able to transmit optical signals in both directions. When an optical signal is transmitted from a node B toward a node D, the optical signal is outputted from the node D via a node C. FIG. 59(b) is a schematic diagram of the BLSR structure when a failure occurs between the node B and the node C. When a failure occurs between the node B and the node C as shown in FIG. 59(b), an optical signal added to the node B is transmitted along a loop in the opposite direction, taking a route along nodes having node IDs B, A, E, D, C, and D, and is dropped from the node D. FIG. 59(c) is a schematic diagram of the BLSR structure in which a failure occurs between the node C and node D. In FIG. 59(c), an optical signal added to the node B is transmitted via B, C, B, A, E, and D (node IDs), and dropped from the node D. Therefore, there are two directions to transmit the optical signal.
Since the BLSR structure can transmit a plurality of optical signals over the same line, it is possible to transmit another optical signal over the protection line, taking advantage of the fact that line switching is not done when the ring transmission system is in the idle state. Namely, the protection line is shared by a plurality of optical signals on the working line and the protection line. For this, line switching may cause a connection (misconnection) that an optical signal is inadvertently transmitted over the plural lines since the protection line accommodates a plurality of lines.
For the purpose of more complicated line switching, each node is equipped with a squelch table (Squelch Table), and a RIP table (Ring Interworking on Protection Table); hereinafter referred to as an RIP table. The squelch table will be now described with reference to FIGS. 60 through 63. The RIP table closely relates to DCP (Drop and Continue on Protection) connection, and DTP (Dual Transmit on Protection) connection representing connection modes of the ring transmission system; the squelch table will be described when DCP connection and DTP connection are explained with reference to FIGS. 64 through 69.
The above squelch is a process performed to prevent an optical signal being now transmitted from being connected to an inappropriate node and to save the optical signal, when failures occur at two or more positions on the ring transmission path and the ring transmission path is thus segmented. In concrete, the squelch signifies an operation performed by each node to add an AIS (Alarm Indication Signal) to the optical signal, thereby avoiding interference of the optical signal. The squelch is also referred to as squelching or squelch process, but they have the same meaning in their use. A state in which the ring transmission line is segmented is occasionally referred to as ring segmentation.
In order to achieve the squelch function, each node is equipped with a squelch table. The squelch table is a table in which line connection information necessary for squelching is written, that is, a table in which data showing an ID of an add node and an ID of a drop node of a line that is currently operated are written. The squelch table is provided for each STS line, by which each node can acquire cross connect information on from which node to which node a path (route) is set. The cross connect information represents a line connect state that an optical signal is to be directed in which direction, or the optical signal is to be passed through or not at a certain node in the BLSR structure.
Each node determines whether squelching should be done on an optical signal or not, specifies a position at which a failure has occurred, and compares the position with the squelch table, thereby determining whether it is possible to save the optical signal or not. When saving the optical signal is impossible, squelching is performed.
On the BLSR-structured ring transmission path, every node needs to accurately know a position on the ring transmission path at which a failure has occurred, and a relation between the position and its own position in order to form a detour for the optical signal in cooperation, as stated above. In order to determine the positional relation, individual node IDs are assigned to the nodes, and each node has the order in which the node IDs have been arranged as topology (ring topology).
FIG. 60 is a diagram showing topology. A ring transmission system 82 in the BLSR structure shown in FIG. 60 has four nodes A, B, C, and D, in which optical signals are transmitted over a working line and a protection line. Each node has E (East) and W (West) in respective directions toward two adjacent nodes. A way to determine the directions is that a direction in which an optical signal is transmitted clockwise is defined as E direction (W→E direction, eastward round, clockwise), and a direction in which the optical signal is transmitted counterclockwise is defined as W direction (E→W direction, westward round, counterclockwise), when the ring transmission system shown in FIG. 60 is looked from the top. Each node has topology (TPLOGY) in which its own node ID always takes the lead. For example, the node A has topology A, B, C, and D in which itself is in the lead.
FIG. 61 is a diagram showing a format of the data link. As stated above, the data link has crossconnect information in which and ID of an add node and an ID of a drop node are written. The ID of an add node is described as source node ID (Source Node ID), whereas the ID of a drop node is described as destination node ID (Destination Node ID).
The data link shown in FIG. 61 has the E→W direction and the W→E direction, separately, and has transmit data and received data in the respective directions.
Each piece of the crossconnect information is managed by one byte; eight bytes when the all are summed up. As each one byte, four bits are assigned to each of a source node ID part and a destination node ID part to manage information on transmission and reception. Whereby, crossconnect is set in the data link, and each node can recognize which is connected to which.
FIG. 62(a) is a schematic diagram of ring transmission paths having add nodes and drop nodes, in which node A, node B, and node C are connected one to another. An optical signal is added to the node C shown in FIG. 62(a), transmitted to the node B over a path 2, and dropped from the node A. On the other hand, an optical signal is added to the node B, transmitted to the node C over a path 1, and dropped from the node C.
FIG. 62(b) is a diagram showing an example of a squelch table of the node B. The squelch table 84 has two paths 1, and 2, in which a source node ID indicating an add node and a destination node ID indicating a drop node are written for each of the paths.
FIG. 62(c) is a diagram for comparing squelch table values of the nodes, where squelch tables 83a, 83b, and 83c are shown. The squelch table 83a shown in FIG. 62(c) corresponds to the west side of the node A, representing that an optical signal on the path 2 added to the node C is to be dropped from the node A. The squelch table 83b corresponds to the east side and the west side of the node B, in which an ID of the add node C and an ID of the drop node A are written in the column of the east side. Besides, an ID of the add node C and an ID of the drop node A in the W→E direction are written in the column of the west side, and an ID of the add node B and an ID of the drop node C in the E→W direction are written in the column of the west side, as well. In the squelch table 83c, an ID of the add node B and an ID of the drop node C in the E→W direction are written in the column of the east side, and an ID of the add node C and an ID of the drop node A in the W→E are written in the column of the east side.
In the squelch table 83b shown in FIG. 62(c), there are eight frames in which Source and Dest (Destination) are entered. Each of these represents an ID in the same position as a corresponding frame among the eight frames in each of the squelch tables 83a, 83b, and 83c. For example, B and C in the E→W entered in the column of the west side in the squelch table 83b represent a source node ID and a destination node ID, respectively.
In other words, an added node enters its own ID in the source node ID part, enters a destination node ID of a received data link in the destination node ID part, and transmits them. On the other hand, a dropping node enters its own node in the destination node ID part, enters a source node ID of a received data link in the source node ID part, and transmits them.
Next, description will be made of a concrete example of data flow in a data link through three nodes with reference to FIGS. 63(a) through 63(f). FIG. 63(a) is a schematic diagram in which three nodes are connected, where the nodes 1, 2, and 3 have node IDs 1, 2, and 3, respectively. FIG. 63(b) is a diagram showing contents of the data link at each node in the initial state, in which the node 1 writes its own absolute node ID in the source node ID part in the E→W direction when the node 1 is add-crossconnected. FIG. 63(c) is a diagram showing contents of the data link at each of the nodes when “add-crossconnect” is set at the node 1. As shown in FIG. 63(c), a node ID of a destination node received by the node 1 is entered in the destination node ID part in the E→W direction of the node 1, and transmitted.
FIG. 63(d) is a diagram showing contents of the data link at each of the nodes when “through” in the E→W direction is set at the node 2, where the node 2 receives a drop node 3, transmits this value, and transmits data received from the E side to the W side as it is.
FIG. 63(e) is a diagram showing contents of the data link at each of the nodes when “drop-crossconnect” is set at the node 3. The node 3 implements drop-crossconnect so as to write its own absolute node ID in the destination node ID part in the E→W direction, copies a received source node ID part in the source node ID part to be transmitted, and transmits them. The node 1 again replaces the destination node ID part when detecting that the received destination node ID part has been changed, and transmits it.
FIG. 63(f) is a diagram showing contents of a squelch table at each of the nodes. As shown in FIG. 63(f), when data on the transmitting side becomes identical to data on the receiving side in the squelch table at each of all the nodes, the node 1 determines that a data link in the ring transmission path is completed, and takes out the data to create a squelch table on the basis of the data link.
There are a number of items to be set when a squelch table is created, it is thus required to decrease the number of items as less as possible. It is also required that setting at each node are automatically done using information about the data link, so that the setting in the optical transmitting apparatus is readily done.
When the number of rings is one, a bandwidth for the protection line is replaced with a bandwidth for the working line when a switching is done. Therefore, it is unnecessary to perform the squelch operation, and to use the data link by the line setting to inform of a state of the line setting.
Even if crossconnect is executed in a bandwidth for the protection line, information in a bandwidth for the working line is transferred to the bandwidth for the protection line whenever a switching is done, there is thus no possibility of occurrence of misconnect as a bandwidth for the protection line. For this, there is no necessity to create a squelch table for a bandwidth for the protection line; a data link for creating a squelch table is not used for a bandwidth for the protection line.
According to the specifications of BLSR, the number of nodes possessed by one ring is 16. When nodes in number more than that is necessary, two or more rings are interconnected. This interconnection system for rings is also referred to as ring interconnection. There are four types of ring interconnection; DCW (Drop and Continue on Working), DTW (Dual Transmit on Working), DCP (Drop and Continue on Protection), and DTP (Dual Transmit on Protection).
Each node can find from a so-called RIP table which type of ring interconnection among the four itself belongs to.
Next, description will be made of an RIP table. The RIP table is a table in which information necessary to connect two or more ring transmission systems is written.
With the RIP table, a ring transmission system in which two or more rings are connected (also referred to as a dual ring transmission system) is realized.
In DCP (DCP application) and DTP (DTP application), optical signals are transmitted over a protection line between a primary node and a secondary node. In DCW (DCW application) and DTW (DTW application), optical signals are transmitted over a working line between a primary node and a secondary node.
a difference between a group of DCP and DTP, or and a group of DCW and DTW is that optical signals are transmitted over the protection line or the working line. The following description will be made by way of DCP and DTP unless otherwise provided.
The DCP connection and DTP connection stand for a connection structure in which, when an optical signal is added to a certain ring transmission system and the added optical signal is dropped to another ring transmission system, there are provided two add nodes, and better one of the added optical signals in two systems is selected at a drop node, and dropped.
Assume that, in a ring transmission system in which a first ring transmission system and a second transmission system are combined, an optical signal transmitted from another external ring transmission system is added to the first transmission system, the added optical signal is transmitted to the second ring transmission system, then the optical signal is dropped therefrom to still another external ring transmission system.
When one node in the first ring transmission system receives the optical signal from another different ring transmission system and transmits the optical signal to the second ring transmission system, two nodes of the second ring transmission system receive the optical signal separately from the first ring transmission system.
The two nodes in the second ring transmission system are called a primary node and a second node. A node in the second ring transmission system that transmits the optical signal from the first ring transmission system to another external ring transmission system is called a drop node.
The optical signal received separately by the primary node and the secondary node is transmitted in two systems in the second ring transmission system, and better one of the optical signals in two systems is selected at the drop node and transmitted to still another external ring transmission system.
The difference between the DCP connection and the DTP connection is in a positional relation between the primary node and the secondary node. In concrete, in the DCP connection, no drop node exists between the primary node and the secondary node, whereas, in the DTP connection, a drop node exists between the primary node and the secondary node.
Hereinafter, DCP and DTP will be described with reference to FIGS. 64 through 69.
FIG. 64 is a schematic diagram of the DCP connection. A ring transmission system 90 and a ring transmission system 91 shown in FIG. 64 are combined by coupling nodes belonging to the respective ring transmission systems, where optical signals can be transmitted in these plural ring transmission systems. In FIG. 64, solid lines represent working lines (expressed as “Work”), while broken lines represent protection lines (expressed as “PTCT”). In a node 90a, there is a service selector SS that transfers data. Additionally, the DCP connection is expressed as “DCP”.
The ring transmission systems 90 and 91 also stand for transmission rings. Accordingly, the ring transmission systems 90 and 91 signify one ring, or two, three or more rings connected to one another. In the following description, the ring transmission system will be occasionally called merely a transmission ring or a ring.
The ring transmission system 90 receives an optical signal at two nodes, a primary node 90a and a secondary node 90b, from the ring transmission system 91. The secondary node 90b transmits the received optical signal to the primary node 90a over the protection line. The primary node 90a selects better one of the same optical signals in two systems received over the working line and the protection line, and transmits one having a better quality to a drop node 90c. The drop node 90c transmits the optical signal to another external ring transmission system.
FIG. 65(a) is a diagram showing a structure of the DCP connection. FIG. 65(b) is a diagram illustrating squelch tables of respective nodes DCP-connected to one another. The DCP connection shown in FIG. 65(a) has a drop node 90c (also referred to as node 1), a primary node 90a (also referred to as node 3), and a secondary node 90b (also referred to as node 5). The drop node 90c is arranged outside a region between the primary node 90a and the secondary node 90b. Optical signals are added to the primary node 90a and the secondary node 90b. One having a better quality of these optical signals in two systems is selected at the primary node 90a, transmitted to the drop node 90c, then transmitted to another ring transmission system (not shown) from the drop node 90c. 
On the other hand, an optical signal added to the drop node 90c is transmitted over the working line and dropped from the primary node 90a, while transmitted over the protection line and dropped from the secondary node 90b. Namely, the optical signal is dropped and continued at the primary node 90a. 
Squelch tables are set only in a region between the node 1 and the node 3 through which the working line passes, as shown in FIG. 65(b). In a region between the node 4 and the node 5 in which the protection line is used, no squelch table is set. In a region between the node 1 and the node 3, node IDs at both ends of a DCP-connected path are set as a source node ID and a destination node ID.
FIG. 66 is a diagram illustrating operations of the primary node and the secondary node in the DCP connection. As shown in FIG. 66, when a failure occurs in the working line involving the primary node, the secondary node executes an add/drop control on the protection line in the opposite direction to the primary node, and writes AIS that is an alarm signal in the protection line on primary node's side. When a failure occurs in the working line not involving the secondary node, the primary node inhibits “continue” of the optical signal to fix the setting of an SS (Service Selector) to add node's side. The secondary node executes “drop and continue” of the optical signal on the protection line in a direction toward the primary node to transmit the optical signal to the primary node. When a failure occurs in the protection line, or when a failure occurs in a span through which no optical signal passes, the secondary node executes an add/drop control on the protection line, and writes an AIS in a line on the primary node's side.
FIG. 67 is a schematic diagram of the DTP connection. A ring transmission system 92 and a ring transmission system 93 shown in FIG. 67 are coupled to transmit optical signals. Incidentally, solid lines shown in FIG. 67 represent working lines (denoted by “Work”), whereas broken lines represent protection lines (denoted by “PTCT”).
The ring transmission system 92 receives optical signals by two nodes, a primary node 92a and a secondary node 92b, from the ring transmission system 93. The primary node 92a transmits the received optical signal to a drop node 92c over the working line, whereas the secondary node 92b transmits the received optical signal identical to the former to the drop node 92c over the protection line. The drop node 92c selects a better one of the identical optical signals in two systems received over the working line and the protection line, and transmits it to another external ring transmission system (not shown). In each of the optical transmitting apparatus 92c and an optical transmitting apparatus 93c, a switch PSW (Path Switch) for selecting and dropping data is shown. Additionally, there is also shown DTP in order to indicate the DTP connection.
FIG. 68(a) is a diagram showing a structure of the DTP connection. FIG. 68(b) is an illustrative diagram of squelch tables of DTP-connected nodes. The DTP connection shown in FIG. 68(a) has a primary node 92a (also referred to as node 1), a drop node 92c (also referred to as node 3), and a secondary node 92b (also referred to as node 5). The drop node 92c is arranged between the primary node 92a and the secondary node 92b. 
To the primary node 92a and the secondary node 92b, optical signals from an external ring transmission system (not shown) are added, and transmitted to the drop node 92c over a working line and a protection line in two systems. Better one of the optical signals in the two systems is selected at the drop node 92c, and transmitted to still another ring transmission system (not shown).
On the other hand, an optical signal added to the drop node 92c from another ring transmission system is transmitted over the working line and dropped from the primary node 92a, while being transmitted over the protection line and dropped from the secondary node 92b. 
Similarly, nodes that are required to set squelch tables are only in a region between the node 1 and the node 3 through which the working line passes. In addition, between the node 4 and the node 5 using only the protection line, no squelch table is set. Node IDs in a PCA (Protection Channel Access) span of a DTP-connected path between the node 1 and the node 3 are set as a source node ID and a destination node ID. The PCA signifies that optical signal are transmitted over the protection line when the line is not switched, which is eroded when the line is switched since the working line uses the protection line, whose priority is low. Namely, on the occasion of a connection to the secondary node, the PCA is used in order to avoid use of the working line.
FIG. 69 is a diagram illustrating operations of the primary node and the secondary node in the DTP connection. When a failure occurs in the working line involving the primary node, the operations are as follows:
Namely, when a failure occurs in the working line involving the primary node, the secondary node continuously executes the add/drop control on the protection line. When a failure occurs in the working line not involving the primary node, the primary node executes a normal switching operation. When the primary node merely passes optical signals therethrough, the primary node operates as a through node. The secondary node executes “drop and continue” of the optical signal on the protection line in a direction toward the primary node to transmit the optical signal to the primary node. Additionally, the secondary node performs setting of a service selector SS on the protection line toward a terminal node. The terminal node is a node having both an add function and a drop function, or a node having only the drop function.
When a failure occurs in the protection line, or when a failure occurs in a span through which no optical signal passes, the primary node carries out a normal switching operation. Only when the primary node merely passes optical signals therethrough, the primary node operates as a through node. The secondary node inhibits “add/drop crossconnect” using the protection line.
When a failure occurs in the working line or the protection line involving a terminal node, the primary node carries out a normal switching operation. Only when the primary node merely passes optical signals therethrough, the primary node operates as a through node. The secondary node inhibits “add/drop crossconnect” using the protection line, similarly.
The DCP connection and the DTP connection are used to connect two or more rings, and further improve use efficiency of the line. In the case of the DCP connection and the DTP connection in four-fiber (or two-fiber) BLSR, it is necessary to configure squelch tables in a form involving a bandwidth for the protection line. For this, the secondary node in the DCP connection and the DTP connection is required to change the line switching operation according to a span in which a failure occurs. Each node thus has to recognize information about in which span the working line is set, whether or not itself is defined as a secondary node in the DCP connection or the DTP connection, then operates.
Namely, in the case of the DCP connection or the DTP connection, an increase in number of nodes relating to a path makes it difficult to express crossconnection information by only the above squelch tables. In order to configure more complicated squelch tables and manage switching of each line, an RIP table for each node is configured so as to cope with switching in a more complicated structure. The RIP table has contents almost identical to those of the squelch table, in which node IDs of a primary, secondary, and terminal nodes are stored. Setting of these is required for each line. The setting has to be done for each node, for each STS line, and for each direction when done by the user.
BLSR technique has been standardized (GR-1230) in the classification of SONET in the specifications of Bellcore (Bellcore Corp.), a way to realize which is known. The above UPSR technique has been standardized in the classification of SONET in the specifications of Bellcore, a way to realize which is known, as well. Hereinafter, description will be mainly made of what relates to this invention; description of others is omitted.
The above BLSR is specifically referred to as normal BLSR (hereinafter expressed as “Normal BLSR” in the drawings).
One of the normal BLSRs, in which an excessive path (path in a redundant part) is eliminated on the occasion of restoration, is specifically called a submarine BLSR. The submarine BLSR is the one in which a cause of quality degradation arising when the normal BLSR is used is eliminated from the normal BLSR.
The submarine BLSR is used as a node that connects continents by, say, a submarine cable. In order to transmit optical signals from West Coast in America to Japan, a submarine cable is laid between Hawaii and Japan. When a failure occurs between Japan and Hawaii, for example, a node in Hawaii changes a direction to transmit optical signals from Japan to Alaska, the optical signals are thus transmitted to Japan via Alaska. The optical signals are transmitted for a distance equal to a round-trip between Hawaii and a position at which the failure has occurred (failure position), so that a loss of the optical signal generates due to an excessive path when the optical signal is looped back.
When a detouring distance is long and becomes tens of thousands of kilometers in total, the optical signals are looped back on the shortest route. When the submarine BLSR is used, the optical signals are looped back at the terminal node, whereby the excessive path is eliminated, which remarkably increases effect of path shortening, restoring the optical signals.
With the normal BLSR and the submarine BLSR, it is possible to improve the coefficient of line utilization and the rate of line saving, coping with an increase in transmission capacity in recent digital optical communications.
Next, description will be made of double-sided and single-sided. Namely, double-sided stands for that both ends (two nodes) of one ring are DCP- (DTP-, (DCW-, or DTW-) connected, whereas single-sided stands for that one end (one terminal node) of one ring is DCP- (DTP-, DCW-, or DTW-) connected.
In case where one ring is connected to two rings, double-sided DCW stands for a connection mode of the central ring when the central ring is DCW-connected to two rings.
On the other hand, in the case where one ring is connected to two rings, single-sided DCW stands for a connection mode of the central ring when the central ring is DCW-connected to one out of the two rings. Namely, it stands for that a position connected to another ring is only one in the ring. Double-sided and single-sided are used in the above sense in the following description.
FIG. 70 is a diagram showing a structure of single-sided DCW. In a ring transmission system 120a shown in FIG. 70, a ring 1 and a ring 2 are connected to each other. Ring 1-node 3 (indicating a node 3 of the ring 1, expressed in the same way in the following description) and ring 1-node 4 make a pair, while ring 2-node 1 and ring 2-node 6 make another pair.
The ring 1 shown in FIG. 70 is in the DCW structure with single-sided nodes 3 and 4. The ring 2 is in the DCW structure with single-sided nodes 1 and 6.
A pair of a primary node (denoted as Primary Node) and a secondary node (denoted as Secondary Node) access to the same optical signal, and transmit the optical signal to another transmission ring.
An optical signal added to a terminal node (ring 1-node 1) passes through ring 1-node 2, reaches ring 1-node 3 that is the primary node, and is split into two at the primary node. One of the split optical signals is directly dropped to the ring 2-node 1, while the other optical signal is continued to the secondary node (ring 1-node 4). The continued optical signal is added to ring 2-node 6 that is the secondary node, and transferred to the ring 2-node 1.
At the primary node (ring 2-node 1) of the ring 2, either one of the optical signal coming in from the ring 1-node 3 and the optical signal coming in from the ring 1-node 6 is selected and transferred to ring 2-node 2, and finally dropped from the terminal node (ring 2-node 3).
Unlike DCP, a path of an optical signal to ring 2-node 6 and ring 2-node 1 from ring 1-node 3 via ring 1-node 4 shown in FIG. 70 is established using the working line in the single-sided DCW. When the optical signal is transmitted over the protection line between the above points, the system is in DCP.
In FIG. 70, a path from ring 1-node 4 to ring 2-node 6 is established using the working line. Therefore, the path using the working line is occupied by the optical signal on the working line although it is redundant. This causes that other optical signal cannot use the same channel on the working line, which in turn causes a decrease in coefficient of line utilization.
With respect to DCW, the line in a single ring is occupied by optical signals on the working line, which leads to a decrease in coefficient of line utilization.
Meanwhile, it is possible to provide another node between ring 2-node 1 and ring 2-node 6.
FIG. 71 is a diagram showing a structure of DTW. In a ring transmission system 120b shown in FIG. 71, optical signals are transmitted over a working line (Working) in dual directions (Dual) by one terminal node in a ring 1 or a ring 2.
Ring 1-node 3 and ring 1-node 4, and ring 2-node 1 and ring 2-node 6 shown in FIG. 71 are connected as primary nodes and secondary nodes, respectively, thereby connecting the rings.
An optical signal added to ring 1-node 1 that is a terminal node is split into two; one to be transmitted on the eastward round (clockwise), and the other to be transmitted on the westward round (counterclockwise). The optical signal from ring 1-node 1 reaches ring 1-node 3 that is the primary node via ring 1-node 2, and transferred to ring 2-node 3 that is the terminal node via ring 2-node 1 and ring 2-node 2 as it is. On the other hand, the optical signal added to the ring 1-node 1 is transferred counterclockwise to ring 2-node 3 via ring 1-node 6, ring 1-node 5, . . . and ring 2-node 4.
Ring 2-node 3 that is the terminal node performs path-switching (Path Switch) on the received optical signal transmitted clockwise and the received optical signal transmitted counterclockwise to finally drop either one of the optical signals.
On the other hand, ring 1-node 1 (terminal node) selects either one of paths of the optical signal transmitted from ring 1-node 3 (primary node) over the working line and the optical signal transmitted in the opposite direction from ring 1-node 4 (secondary node) over the protection line, and drops either one having a better quality to transmit it to another ring. An optical signal added to the ring 1-node 1 (terminal node) is connected to ring l-node 3 (primary node) over the working line, while connected to ring 1-node 4 (secondary node) in the opposite direction.
In DTW, the service selector control is not performed at ring 1-node 3 (primary node), and a direction of adding/dropping at ring 1-node 4 (secondary node) is opposite to that in DCP. Similarly to DCW described above, an optical signal transmitted on the eastward round and an optical signal transmitted on the westward round are both transmitted over the working line in DTW.
Next, description will be made of line connection modes using RIP tables with reference to FIGS. 72 and 73. FIG. 72 is a diagram showing a structure of single-sided DCP. A ring transmission system 120c shown in FIG. 72 is DCP-connected at terminal nodes (ring 1-node 3, ring 1-node 4) in a ring 1, while DCP-connected at other terminal nodes (ring 2-node 1, ring 2-node 6) in a ring 2, as well.
Ring 2-node 1 shown in FIG. 72 drops an optical signal from ring 2-node 3 (terminal node) over the working line, and continues the optical signal toward a secondary node (ring 2-node 6). The secondary node (ring 2-node 6) drops the optical signal over the protection line.
On the other hand, ring 2-node 1 is inputted thereto an optical signal transmitted from ring 1-node 3 over the working line and an optical signal transmitted from the ring 2-node 6 over the protection line, selects either one of them by a service selector SS, and transmits the selected optical signal to ring 2-node 2 over the working line.
This single-sided DCP differs from the single-sided DCW (refer to FIG. 70) in that the optical signal transmitted from the ring 1-node 3 to ring 2-node 6 and ring 2-node 1 via ring 1-node 4 is transmitted over the protection line. Namely, the optical signal is transmitted from ring 1-node 3 to finally ring 2-node 1 over the working line. When the rings are interconnected using RIP tables, it is possible to transmit another optical signal over the working line in a span interconnected over the protection line. For this, the single-sided DCP has a higher coefficient of line utilization than the single-sided DCW.
Namely, a difference between the single-sided DCP and the single-sided DCW is in interconnected portions between the primary nodes and the secondary nodes of ring 1-node 3 and ring 1-node 4, and ring 2-node 1 and ring 2-node 6, respectively.
Incidentally, in FIG. 72, another node may be installed between ring 2-node 1 and ring 2-node 6.
FIG. 73 is a diagram showing a structure of DTP. A ring transmission system 120d shown in FIG. 73 is DTP-structured at one terminal node in a ring. Namely, in the ring 1, ring 1-node 1 (terminal node) selects a path of either an optical signal transmitted from ring 1-node 3 (primary node) over the working line or an optical signal transmitted in the opposite direction from the ring 1-node 4 (secondary node) over the protection line, and drops an optical signal on the selected path.
On the other hand, an added optical signal is connected to ring 1-node 3 (primary node) over the working line, while being connected to ring 1-node 4 (secondary node) over the protection line.
The above is similar in a ring 2. Ring 2-node 3 (terminal node) selects a path of either an optical signal from ring 2-node 1 (primary node) over the working line or an optical signal in the opposite direction from ring 2-node 6 (secondary node) over the protection line, and drops an optical signal on the selected path. An added optical signal is connected to ring 2-node 1 (primary node) over the working line, while being connected to ring 2-node 6 over the protection line.
A difference between DTP and DCP is in positional relation between the primary node and the secondary node; DCP is a connection without a drop node between the primary node and the secondary node, whereas DTP is a connection with a drop node between the primary node and the secondary node. Unlike DTP, a service selector SS is provided in the primary node in DCP. In DTP, an optical signal in the counterclockwise direction added from ring 1-node 1 is transmitted over the protection line, unlike DTW.
Namely, in the ring 1, ring 1-node 3 and ring 1-node 4 are DTP-connected, whereas ring 1-node 1 and ring 1-node 6 is not DTP-connected.
As above, when either one of optical signals in two systems added to two add nodes is selected and dropped, and the rings are interconnected using RIP tables, a line span interconnected using the protection line can be used for transmission of another optical signal using the protection line, which leads to a higher coefficient of line utilization than that of DTW.
Next, description will be made of a loop-back control when a failure occurs in the normal BLSR with reference to FIGS. 74 and 75.
FIG. 74 is an illustrative diagram of transmission as normal BLSR in normal operations (in normal time in the Normal-BLSR). An optical signal added from the W (West) side to a node C in a ring 200 shown in FIG. 74 is transmitted counterclockwise, finally reaches a node F via a node B and a node A, and is dropped from the E (East) side of the node F. To the contrary, an optical signal added from the E side of the node C is transmitted clockwise to the node D, and dropped from the W side of the node D.
Optical signals in the normal BLSR are transmitted over the same working line in the ring 200, so that the normal BLSR has a higher coefficient of line utilization than the UPSR.
FIG. 75 is a diagram illustrating a line restoration control (restoration control in the Normal-BLSR). FIG. 75 shows flow of optical signals under a control for line restoration when a failure occurs between the node A and the node B in the ring 200 shown in FIG. 74.
When the failure occurs, the node A and the node B shown in FIG. 75 loop-back optical signals as failure nodes situated on both sides of the failure position. Line information is transmitted counterclockwise from the node C toward the node F shown in FIG. 75.
The optical signal is looped-back at the node B by a bridge from the working line to the protection line, passes through the node C, the node D, the node E, the node F, and the node A, again is looped-back toward the node F, then is dropped from the E (East) side of the node F. An optical signal being transmitted from the node C toward the node D is continuously transmitted as it is since the optical signal is not affected by the failure.
The optical signal transmitted from the node C to the node F passes through extra paths until reaching the node F. These extra paths are a path between the node B and the node C, and a path between the node A and the node F shown in FIG. 75, which are detouring routes.
As above, there are many kinds of modes of ring connection. It is required that optical signals be transmitted not only safely but also for the shortest distance between nodes configuring the ring.
Meanwhile, as types of ring connection, there are connections between DCW (own node) and DCP (opposite node), DCW (own node) and DCW (opposite node), etc. However, the loop-back control method does not differ depending on only a type of the connection.
Further, as another mode of connection, there are a primary node and a secondary node of own nodes and the opposite nodes. When rings are interconnected, the primary node has two optical signals; one is dropped from a primary node or a secondary node of another ring to be added to its own ring, and the other is dropped from the primary node or the secondary node of another ring, added to a secondary node of its own ring, and reaches the primary node over the working line or the protection line.
The service selector function selects one having a better quality of the former and latter optical signals, and transmits the selected optical signal to a node that is an object of the transmission. The technique regarding the service selector function in the BLSR is described in standards of SONET, thus is known; detailed description of which is omitted.
Next, description will be made of a loop-back control with respect to transmission for the shortest distance by way of example where the number of rings is three in the case of connections between DCW (own node) and DCP (opposite node), and DCW (own node) and DCW (opposite node).
FIG. 76 is an illustrative diagram of a DCP-DCW inter-ring connection, showing a modified connection of double-sided DCP. One side of a ring 2 shown in FIG. 76 is DCP-connected to a ring 1, the other side of the ring 2 is DCW-connected to a ring 3, and ring 2-node 4 (secondary node) in the DCP structure and ring 2-node 6 (secondary node) in the DCW structure are connected over the protection line, the working line and the working line. Before a failure occurs, an optical signal is added to ring 3-node 1 and dropped from ring 1-node 3.
First, an optical signal is added to ring 3-node 1, and transmitted to ring 3-node 3. From node 3, one of the split optical signals is transmitted to ring 2-node 1, while the other is transmitted to ring 2-node 6 via ring 3-node 4 (secondary node), then to ring 2-node 1 (primary node).
At ring 2-node 1, either one of the optical signals coming in from the two directions is selected by a service selector SS, and the one having a better quality is transmitted to ring 2-node 3 via ring 2-node 2. One of the optical signals split at ring 2-node 3 is transmitted to ring 1-node 1, whereas the other is transmitted to ring 1-node 1 via ring 2-node 4 and ring 1-node 6. The optical signal is transmitted by a service selector SS to ring 1-node 3 from the ring 1-node 1, then dropped therefrom. The optical signal is transferred over the working line between ring 2-node 1 (primary node) and ring 2-node 6 (secondary node).
In FIG. 76, it is possible to provide another node between ring 2-node 3 and ring 2-node 4, or ring 2-node 1 and ring 2-node 6.
How the line restoration is controlled when a failure occurs in the working line will be next described, focusing on a portion in which a transmission route of the optical signal is changed due to the failure.
Description will be first made of a detouring route established when a failure occurs with reference to FIG. 77. FIG. 77 is a diagram illustrating a ring at the time of failure occurrence, where a failure occurs in a ring transmission system 500 having a ring switch function. FIG. 77 shows a DCP-DCW connection, in which it is assumed that a failure occurs between a primary node (ring 2-node 1) and a secondary node (ring 2-node 6) corresponding to opposite nodes of ring 2-node 3 and ring 2-node 4, respectively. In this example, a region between DCP (own nodes) and DCW (opposite nodes) shows line restoration in a known apparatus.
When a failure occurs between ring 2-node 1 and ring 2-node 6, an optical signal transmitted from ring 3-node 4 to ring 2-node 6 is ring-bridged due to the occurrence of failure, and looped-back over a protection line on the westward round. Accordingly, the path of the optical signal is changed at ring 2-node 6.
The optical signal is next transferred to ring 2-node 3 via ring 2-node 5 and ring 2-node 4. The optical signal transferred over the protection line on the westward round (counterclockwise) is switched to a working line on the eastward round (clockwise) by a ring switch in ring 2-node 3 to be restored.
The optical signal looped-back at ring 2-node 6 and transmitted to ring 2-node 5 and ring 2-node 4 over the protection line is dropped (DTP-switch-drop-West) from ring 2-node 4, then added to ring 1-node 6. The added optical signal is further transferred to ring 1-node 1, controlled by a service selector in ring 1-node 1, transferred to ring 1-node 2 and ring 1-node 3, and dropped therefrom.
In the BLSR structure, the number of nodes in one network is limited up to 16 according to byte limitation of APS signal. Additionally, four bits are used to represent an absolute node ID in the present data link, so that all data (0 to 15) that can be expressed by four bits is used to indicate a node ID; no region for representing additional information is left therein. It is therefore impossible to use a flag or the like to transmit information in order to notify of the working line and a use span at the secondary node, so that information used to normally carry out a switching process cannot be notified.
When line setting is executed and each piece of information is notified, each node cannot recognize at which timing data on the data link is changed. It is therefore necessary for each node to recognize completion of a change in the squelch table to prevent beforehand a misconnect that may occur due to a switching. When plural pieces of information is notified in the data link, each node cannot recognize when a change in the data link is completed.
In addition, when a failure occurs while a data link is under establishment, each node cannot know that the data link is not transmitted beyond a certain node. Each node thus cannot recognize whether or not the all nodes normally receive the data link and squelch tables are created.
As a result, six problems arise as follows: First, when each node creates a squelch table and an RIP table and carries out crossconnect-setting in the DCP connection and the DTP connection, values of the data link are always varied. In this case, the terminal node has to create a squelch table without using a node ID of its own in some occasion, thus cannot clearly determine whether the data link is completed or not. The terminal node also has to create data including the protection bandwidth. The current manner has a problem that each node cannot notify of a state of setting of the protection bandwidth.
Second, the RIP table and the squelch table are incompatible although having similar features. Hardware of these optical transmitting apparatus is suited to create the squelch tables, but not suited to create the RIP table having a large amount of information. However, a new apparatus is sometimes connected to an old one in a ring system; it is impossible to newly produce the hardware.
Third, a meaningless value does not exist among values that can be expressed with four bits even when information on two or more nodes is attempted to be notified a secondary node, so that it is impossible to notify of which value indicates which in information changing momentarily. For this, the normal system operation is impossible unless the user performs various setting for each STS line. Further, when a switching occurs before squelch data is completed, the squelch operation does not normally carried out; which requires an alarm to be notified each node in order to make it known to every node. In this case, each node cannot recognize when the squelch table and the RIP table are completed, so that it is impossible to notify of occurrence or cancellation of the alarm.
Four, the system is configured in complicated operations, which requires a number of settings in order to normally operate the optical transmitting apparatus, leading to a difficult-to-use optical transmitting apparatus. If the measurement is made by changing an amount of information of the data link, it is essential to reform the hardware of the optical transmitting apparatus being now shipped, and to replace the hardware of the optical transmitting apparatus in operation.
Further, there are fifth and sixth problems.
When a failure occurs in the line restoration control shown in FIG. 75, the optical signal passes through the redundant paths, leading to transmission delay and degradation of the optical signal. The degradation of the optical signal in turn requires to increase the number of repeaters to be installed, leading to an increase in cost. For this, there is a demand for an apparatus having a function of being able to loop-back for the shortest distance. Ring 2-node 3 shown in FIG. 77 selects the optical signal transmitted from ring 2-node 4 over the protection line by switching the ring switch. The optical signal received via ring 3-node 3 (primary node), ring 2-node 1 (primary node), and ring 2-node 2 is not selected at all; an optical signal having a high quality cannot be received.
FIGS. 78 and 79 are diagrams for illustrating failures occurring in networks without the ring switch function. A failure occurs between ring 2-node 1 and ring 2-node 6 shown in FIG. 78.
An optical signal from ring 3-node 4 is ring-bridged at ring 2-node 6, transmitted over a protection line, and ring-switched at ring 2-node 1 that is a failure detecting node. The optical signal is controlled by a service selector at ring 2-node 1, sent back to ring 2-node 2, and dropped from ring 2-node 3 to ring 1-node 1. At ring 2-node 3, the optical signal received from ring 2-node 2 is unconditionally selected, and transferred to ring 1-node 1.
Fifth, when the optical signal is looped-back at the failure detecting node (ring 2-node 1) as above, the optical signal passes through a redundant path over the protection line from ring 2-node 4 to ring 2-node 1, causing degradation of quality of the optical signal.
In FIG. 79, when a failure occurs between ring 2-node 4 and ring 1-node 6, an optical signal being dropped for the shortest distance from ring 2-node 4 loses its path. Accordingly, the optical signal is outputted from ring 2-node 4, travels around almost the entire protection line of the ring 2, is looped-back at ring 2-node 1, and is dropped from ring 2-node 3 to ring 1-node 1. As a result, the optical signal passes through a redundant path in order to restore the line.